Terminal deoxynucleotidyl transferase (TdT) is uniquely localized in thymus and bone marrow lymphocytes. This project will examine the specific biochemical functions, embryological sources and progeny biological functions of prelymphocytes that differentiate through TdT+ stage. The experiments involve rat, mouse and chicken cells from embryonic tissue and lymphoid organs. Attempts will be made to stimulate TdT- cells from embryo sources to positivity and TdT+ cells from postnatal animals to TdT- functioning lymphocytes. The biochemical consequences of the presence of TdT in cells will be examined at the DNA level. The results should provide information about the role of terminal transferase in development of the immune system.